All existing two-stoke engines suffer from the common drawback that scavenging stroke and exhaust stroke occur in the same space and at the same time which will undoubtedly result in an undesirable but unavoidiable defect that a portion of the scavenging mixture of fuel and air ( hereinunder referred as "the mixture") will be discharged to the environment together with the combustion exhaust and thus reduce the combustion efficiency of the engine. Besides, if the exhaust port is closed before the interface of the mixture and the exhaust reaches it, part of the exhaust will be left behind in the combustion chamber and will inevitably reduce the scavenging efficiency. On the contrary, if the exhaust port remains open after the interfacing gas has passed through it, then, much mixture will, undesirably, escape into the environment together with the exhaust, thus reducing the combustion efficiency of the engine. Accordingly, it has, hitherto, been quite a difficult task to achieve simultaneously an optimum level of both the combustion efficiency and scavenging efficiency in a two-stroke engine.
Specifically, it is hard to catch the aforementioned interface of the mixture and the exhaust because the interface fluctuates according to different temperature, pressure and flow speed. Besides, the shape and length of the passage for scavenging mixture will also affect the speed for the interface to reach the exhaust port. In order to achieve a more complete scavenging effect, several scavenging ports and several scavenging passages are usually provided in the cylinder wall. However, since each current of scavenging mixture flows from one side to the other within the cylinder bore through a comparatively long distance (see FIG. 5 ), the shape and length of the various scavenging mixture currents vary considerably. Consequently, it is difficult to make all the various interfaces between the exhaust and each individual current of scavenging mixture to reach the exhaust port simultaneously so as to decide on the optimum best timing for opening and closing the exhaust port. By conducting numerous experiments and formulating subsequent design improvements in scavenging ports and scavenging passages, it is barely possible to make all the interfaces arrive at the exhaust port simultaneously at a selected engine RPM. However, this will never be possible for all engine revolutions.
In addition, according to the design of a conventional two-stroke engine, it is also difficult, if not impossible, to increase the power output and the combustion efficiency of the engine at the same time. This is due to the fact that the necessary requirement for increasing the power output is to increase the torque of the crank shaft at high engine revolution. However, since the "time area" of the exhaust port--the integral of the opening area of the exhaust port with respect o its opening time--must increase with each increment of the engine revolution, the exhaust port must be provided at higher position for higher RPM engine so that the exhaust port can be opened earlier, which will undoubtedly reduce the effective explosion thrust and, consequently, the combustion efficiency of the engine.
In view of the above-described drawbacks of conventional two-stoke engines, the primary object of this invention is to provide a two-stroke engine in which the scavenging operation is performed by the use of a single current of scavenging mixture which jets upwardly, from the central portion of the cylinder, into the top portion of the cylinder bore, and then spreads radially and outwardly. Since each current of scavenging mixture, branching from the central main stream of scavenging mixture, flows through a relatively short distance to the exhaust port and since each current is subject to similar conditions in its flow environment and flow passage length due to central symmetry, it is possible to make all the various interfaces between the exhaust and each current of scavenging mixture to reach the exhaust port at the same time. Besides, since the upward-flowing scavenging mixture and the downward-flowing exhaust have exactly opposite flow directions, clear-cut interfaces are expected to be formed between them which will greatly enhance the scavenging efficiency of the engine.
Another object of this invention is to provide a two-stroke engine in which the exhaust ports can be disposed at lower location, as compared with conventional two-stroke engines, due to the fact that each exhaust port has relatively larger opening area, the exhaust has relatively shorter flow passage to the exhaust port, as 15 well as the exhaust can be discharged more quickly. This makes it possible to enhance the effective explosion thrust and to minimize the mixture loss through the exhaust port. Accordingly, this invention can maximize the power output of the two-stroke engines without the corresponding reduction in their combustion efficiency.
Yet another object of this invention is to provide a two-stroke engine in which a combustion chamber is provided at the top portion of its piston. Cold mixture comprising air and atomized fuel drops is forced into the top portion of the cylinder through the combustion chamber during a scavenging stroke, and is later forced to return to the combustion chamber during a compression stroke. Accordingly, the piston can be cooled twice and also the fuel and air can be mixed twice, which will improve the cooling effect of the hot piston and enable a more homogeneous mixing of fuel and air. In addition, since the fuel droplets have the chance to be preheated by the hot piston so as to vaporize more completely before combustion, and since air and fuel can be more homogeneously mixed, more complete combustion at high engine revolution can thus be achieved.